A vacuum pump is a device for reducing the pressure of a gas, such as air, within a container below atmospheric pressure so as to increase the vacuum within the container. In practice, where high vacuum is required, two or more pumps are often used in series. For example, a rotary air seal pump may be used in conjunction with a diffusion pump to provide continuity in pumping from atmospheric pressure to a high vacuum range.
Conventional diffusion pumps known in the art are generally comprised of a multi-stage nozzle system, a cylindrical vessel containing the nozzle system and, oftentimes, baffling material to prevent back diffusion. The normal manner of operating diffusion pumps of the type known in the art is to create jets of a pumping liquid (liquids most commonly used being oil or mercury vapor) which strike the water-cooled wall of the vessel and collect in a pool in a boiler at the lowermost portion of the diffusion pump. In this process molecules above the jet streams are trapped in the jet streams and carried out of the pump, thereby evacuating the chamber above the pump.
The working fluid in the diffuser pump is vaporized in a boiler at the bottom of the containment vessel by an electric heater, and it is usual in the prior art to provide a thermostat switch in circuit with the heater to protect the pump from excessive heating. However, as pointed out in U.S. Pat. No. 3,282,330, the usual prior art thermostat control is not responsive to all of the different ways in which the diffusion pump may malfunction.
Specifically, there are three major ways in which the diffusion pump may malfunction, these being: failure in the circulation of cooling water through the coils surrounding the pump containment vessel; excessive pressure rise within the pump, and loss of the working fluid within the pump.
The usual prior art thermostat control systems consist of a thermal switch mounted on a boiler plate forming the bottom of the boiler, and this switch responds only to a rise in the temperature of the boiler plate. With such a prior art protective system, although the thermostat will respond if the wall cooling means fails, its response to the other two sources of malfunction of the pump are less certain.
In the system described in U.S. Pat. No. 3,282,330, the thermal switch is made to be more responsive to the various possible malfunctions of the pump. This is achieved by mounting the thermal switch on an elongated thermal conductor. The conductor is connected at one end to the cooling coils of the pump, and is connected at its other end to the boiler plate at the bottom of the boiler. Such an arrangement is intended to provide a progressive temperature rise from the cold zone of the pump to the hot zone, and is intended to cause the thermal switch to respond to temperature changes in either the cold zone or the hot zone, so that it will be sensitive to any of the malfunctions described above. However, the provision of such an elongated thermal conductor, connected at one end to the boiler plate and at the other end to the cooling coils, provides a direct heat conductive path from the hot zone to the cold zone of the pump, and impairs to a large extent the efficiency of the pump. Also, the mounting of the thermal switch on such a conductor means that the switch must operate at relatively high temperature levels.
The system of the present invention is similar in some respects to the system described in the patent, in that a thermal switch is mounted on a bracket attached at one end to the cooling coils of the pump. However, in the system of the invention, the other end of the bracket, instead of being connected to the boiler plate at the bottom of the boiler, is displaced from the heating block containing the heater, and is heated by radiation and convection from the heater block, so as to minimize the direct transfer of heat from the hot zone to the cold zone of the pump, and also to permit the thermal switch to operate at relatively low temperature levels.
Thus, the system of the invention protects the diffusion pump from loss of water cooling as well as from overheating of the heater, without significantly adversely affecting the normal efficiency of the pump, and by permitting the thermal switch to operate at efficient temperature levels, for example, 260.degree. F. range, as compared with a 600.degree. F. range of the switch in the system of the patent. The thermal switch used in the system of the invention is conductively cooled through its contact through the bracket to the cooling coils of the pump, and it is heated by radiation and convection from the heater. Therefore, overheating of the heater for any reason causes the thermal switch to open, and loss of water cooling will also cause the thermal switch to open, thereby preventing damage of the pump due to any malfunction.
The system of the invention is also useful in conjunction with apparatus other than diffusion pumps, such as distilling apparatus and the like.
The primary purpose of the protective system of the invention, when used in conjunction with a diffusion pump, is to prevent overheating of the heater upon loss of pumping fluid within the pump. In normal operation of the pump, the pumping fluid is vaporized through a jet structure within the containment vessel, and heat is removed from the bottom of the vessel by the transfer of the fluid in vapor form to the cooled walls of the vessels where it is condensed and returned to the boiler at the bottom of the containment vessel.
As mentioned above, the protective system of the invention has additional structural features which, together with the structure described briefly above, will be described in detail in the following specification.